1. Field of the Invention
The present invention relates to an encoder unit and more particularly to an encoder unit using a magnetoresistance effect element in order to detect a the relative position between an object to be detected and a detector, the velocity of an object to be detected and so on.
2. Description of the Prior Art
Magnetoresistance effect elements (to be referred to as "MR elements" hereinafter in this specification) can detect a variation in an applied magnetic field in terms of the variation in electrical resistance of a magnetic thin film element such as Ni-Fe, Ni-Co or the like. Therefore, when sensors using MR elements are used as various encoders, it suffices only to provide a magnetic-field generating means such as a permanent magnet so that encoder units which are simpler in construction and have a higher degree of durability as compared with a photosensor which inevitably requires an electric power supply and a consumable light source such as a light-emitting diode can be provided. Recently office equipment such as magnetic disc devices, printers and the like have been increasingly used in various fields so that the demand for rotary encoder units used for controlling rotational speed or detecting a position has been remarkably increased. At present, almost all rotary encoder units are of the optical type, but the use of magnetic rotary encoder units has been also increased because, in addition to the above described advantages that they are simple in construction and have a high degree of durability, have a higher resolution, a faster response time and a high degree of reliability in case of accumulation of dust particles and frost and are inexpensive to manufacture.
FIG. 1 is a perspective view illustrating a prior art rotary encoder unit. The rotary encoder unit comprises a rotor 1, mounted on a rotating shaft 2 coaxially therewith having a cylindrical surface which is divided into a plurality (three in FIG. 1) of tracks 3A-3C and a detector 4 composed of a plurality of MR sensors 4A-4C which are mounted on a base plate 5 in opposed relationships with the respective tracks 3A and 3C. The tracks 3A-3C have magnetization patterns magnetized in the opposite directions at different pitches, respectively, and the leakage fluxes from the magnetization patterns are detected by MR sensors so that the absolute position and rotational speed of the rotor 1 can be determined. A circuit as shown in FIG. 2 is used to detect a variation in the magnetic field resulting from the rotation of the rotor 1. A resistor R.sub.1 in FIG. 2 corresponds to the resistance of MR sensors 4A-4C shown in FIG. 1 and is connected in series with a temperature compensation resistor R.sub.2 whose temperature coefficient of electrical resistance is equal to that of the resistor R.sub.1. The resistors R.sub.1 and R.sub.2 are connected to resistors R.sub.3 and R.sub.4 across which is applied a voltage of a power supply E so as to establish a reference voltage, whereby a bridge circuit is provided. A differential amplifier A detects the voltage difference between voltage dividing points in the bridge circuit so that a variation in a magnetic field is derived as an electrical signal.
FIG. 3 shows the leakage magnetic fluxes from the magnetization pattern in the cases of the track 3A. The density of leakage fluxes 6 at the center of the magnetization pattern is lower than that at the ends of the magnetization pattern. When the magnetization patterns having various period are provided in order to detect the absolute position of the rotor 1 as shown in FIG. 1, the longer the period of a magnetization pattern, the lower the density of the leakage fluxes becomes at the center of the magnetization pattern. In the case of a magnetization pattern having a long period as shown in FIG. 4, the MR sensor can detect the magnetic fields at the end portions X, but cannot detect the magnetic field at the center portion Y. As a result, the prior art rotary encoder unit of the type described above has the disadvantage that when the rotor 1 is stopped at a position in which the center portion Y of the magnetization pattern is in opposed relationship with the MR sensor, it becomes impossible to accurately detect the position of the rotor 1.
FIG. 5 shows another prior art encoder unit. In FIG. 5, reference numeral 11 represents a motor for controlling the rotation and a magnetic drum 12 whose cylindrical surface is made of a magnetic medium is directly connected to the shaft 11A of the motor 11. Each of tracks 13A-13D on the magnetic drum 12 has a predetermined gray code pattern consisting of magnetized and demagnetized regions. A detector 14 including MR sensors 14A-14D and disposed adjacent to the cylindrical surface of the drum 12 detects a pattern and delivers an electrical signal in response to which the rotation of the motor 11 is detected.
FIG. 6A shows an example of a four-bits gray code recorded on the magnetic drum 12 shown in FIG. 5. FIG. 6B shows a development of the cylindrical surface of the magnetic drum 12 and the hatched areas are regions in which are recorded magnetization patterns which are repeatedly reversed in magnetization at a predetermined pitch while the white areas are the regions which are not magnetized (or which are demagnetized).
In the prior art rotary encoder unit of the type described above, a magnetized region 31 and an unmagnetized region 32 are defined as shown in FIG. 7. Therefore, at the boundary 33 between these two regions 31 and 32, the magnetic flux distribution 34 is widely expanded as compared with the magnetic flux distribution 35 at the portions which are continuously magnetized. Therefore, the prior art rotary encoder units cannot attain a satisfactory accuracy in detection.
As shown in FIG. 6B, in the case of the formation of a gray code, the unmagnetized regions and the magnetized regions must be juxtaposed in the adjacent tracks. In this case, in order to avoid interference between the magnetic fields, the adjacent tracks must be spaced apart from each other by a predetermined distance so that it is difficult to provide a magnetic drum which is compact in size.
FIG. 8 shows a further prior art rotary encoder unit. Reference numeral 41 represents a rotor whose cylindrical surface has a magnetization pattern 42. The magnetization pattern 42 includes an incremental layer 42A used for detection of the direction of rotation, the angle of rotation and the rotational speed of the rotor. The magnetization pattern 42 is comprised of a magnetization pattern at a predetermined pitch and an index layer 42B used for the detection of a reference position of the rotor. MR sensors 43A and 43B are mounted on a base 45 of a detector 44 in opposed relationships with the incremental layer 42A and the index layer 42B, respectively, and are spaced apart therefrom by a predetermined distance.
In general, the electrical resistance of an MR element varies as a function of the angle between the direction of magnetization and the direction in which current flows through the element. That is, the resistance R(.theta.) is given by EQU R(.theta.)=Ra sin.sup.2 .theta.+Rb cos.sup.2 .theta.
where Ra is the electrical resistance of the MR element when a magnetic field is applied in parallel with the current;
Rb is the electrical resistance of the MR element when a magnetic field is applied perpendicular to the direction of the current; and PA1 .theta. is the angle between the direction of magnetization and the direction of the current. PA1 an object to be detected which has a plurality of tracks each having a first region having a magnetization pattern at a predetermined pitch and a second region having no periodic magnetization pattern; and PA1 a detector for detecting the object to be detected having a plurality of sensors using a magnetoresistance effect element disposed in opposed relationship with the object to be detected and oriented in the direction which intersects the direction of the magnetic flux of the magnetized pattern of the first region. PA1 an object to be detected having a track formed with a first magnetization pattern and a second magnetization pattern whose direction of magnetization can be different from that of the first magnetization pattern; and PA1 a sensor for detecting the object to be detected having a magnetoresistance effect element disposed in opposed relationship with the object to be detected and oriented in the direction which intersects the direction of the magnetic flux of the first magnetization pattern. PA1 p' is the pitch of the second magnetization pattern, and PA1 i is an integer except zero. PA1 an object to be detected having a first track with a first magnetization pattern and a second track with a second magnetization pattern consisting of a first magnetized region for detecting a reference position and a second magnetized region which have a recording wavelength shorter than that of the first magnetized region and which is disposed on both sides of the first magnetized region in such a way that the same poles of the first magnetized region and the second magnetized regions are located adjacent to each other; PA1 and a detector having a plurality of magnetoresistance effect elements disposed in opposed relationship with the object to be detected and oriented in the direction which intersects the directions of the magnetic fluxes of the first and second magnetized pattern. PA1 an object to be detected on which surface a track having a first magnetized pattern with first period of repeated magnetization and a second magnetized pattern with the second period shorter than the first period of repeated magnetization is provided; and PA1 a detector for detecting the object to be detected having a magnetoresistance effect element disposed in opposed relationship with the object to be detected and oriented in the direction which intersects the direction of the magnetic fluxes of the first magnetized pattern.
In the case of an encoder unit, an MR element has a unidirectional anisotropy and a magnetic field H is applied perpendicular to the direction of the current I flowing through the element so that the variations in the magnetic field are obtained in terms of the variations in resistance of the MR element.
The magnetic flux distributions in the index layer 42B and the incremental layer 42A shown in FIG. 8 are shown in FIG. 9. In the incremental layer 42A, the direction of magnetization is continuously reversed so that the same magnetic poles are adjacent to each other. As a result, the magnetic flux 46 becomes narrow because of the repulsion between the same poles. The index layer 42B has only one magnetization pattern with the S and N poles so that the magnetic flux 47 is expanded. Therefore, the detection signals obtained by the MR sensors 43A and 43B of the type described above become as shown in FIGS. 10A and 10B. FIG. 10A shows the detection signal obtained in response to the reproduction output of the MR sensor 43A while FIG. 10B shows the detection signal obtained in response to the reproduction output of the MR sensor 43B. The waveform of the detection signal obtained by the MR sensor 43B for detecting the index layer is expanded as compared with the waveform of the detection signal expected to be obtained from the magnetization pattern of the index layer 42 as shown in FIG. 10C. When the waveform of the signal detecting the index layer 42B is widened as described above, there arise the problem that an error occurs in timing for detecting an index pulse depending upon the direction of rotation of the rotor 41.
It is preferable that the output waveform of the index layer is similar in resolution to the output waveform from the incremental layer as shown in FIG. 10C, and it is preferable that the output waveform derived from the index layer become a pulse whose pulse width is narrower than that of the pulses derived from the incremental layer.